With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 50%. In recent years, an information recording capacity exceeding 320 gigabytes per platter has been desired for a magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like. To fulfill such demands, an information recording density exceeding 500 gigabits per square inch is desired to be achieved.
Important factors for increasing recording density of the perpendicular magnetic disk include, for example, an improvement in TPI (Tracks per Inch) by narrowing the track width, ensuring electromagnetic conversion characteristics, such as a Signal-to-Noise Ratio (SNR) and an overwrite (OW) characteristic at the time of improving BPI (Bits per Inch), and further ensuring heat fluctuation resistance with recording bits decreased due to the above. Among these, an increase in SNR in a high recording density condition is important.
In a magnetic layer of a granular structure, which goes mainstream in recent years, a nonmagnetic substance having an oxide as a main component is segregated around magnetic particles having a CoCrPt alloy growing in a columnar shape to form a grain boundary part. In this structure, since the magnetic grains are separated from each other, noise is reduced, and this is effective for a high SNR. An important factor to further increase the SNR is to advance microfabrication of crystal particles and equalize particle diameters (which are collectively referred to as “particle diameter control”), and to improve crystal orientation. Co takes a hcp structure (a hexagonal close-packed crystal lattice structure), and a c-axis direction (an axial direction of a hexagonal column as a crystal lattice) serves as an axis of easy magnetization. Therefore, by orienting the c axis of each of more crystals in a more perpendicular direction, noise is reduced and signals become strong, which can cause an increase in SNR as a synergy effect.
When a film of crystal is formed on crystals by sputtering, as the film thickness is thicker due to epitaxial growth, crystal orientation tends to be improved. Thus, a perpendicular magnetic recording layer is microfabricated and its particle diameter is equalized from an initial growth stage and, in order to improve crystal orientation, it has been conventionally performed that a film of a ground layer (which is also called an intermediate layer) is formed of Ru, which is a metal having a hcp structure, and a film of a perpendicular magnetic recording layer is formed thereon. Furthermore, a crystalline preliminary ground layer (which is also called a seed layer) provided under the Ru ground layer, and crystal orientation of the Ru ground layer is improved.
Patent Document 1 discloses an orientation control layer (a seed layer) having Ni as a main material and having an oxide added thereto. According to Patent Document 1, by using a material added with an oxide as an orientation control layer, the size of magnetic particles can be microfabricated without decreasing magnetic particle density in the magnetic layer. Also, a magnetic layer can be laminated almost without degrading crystal orientation of magnetic grains, and recording and reproducing characteristics of a perpendicular recording medium can also be improved.